The accumulation of heavy toxic metals in wastewater sludges has long been recognized as a major problem in sludge disposal. The origin of the mineral values may variously be industrial discharges, urban runoff and sewage, or simply a high natural mineral content in the water itself. Moreover, it is apparent that as a result of the rising dollar costs of metals contained in these wastewaters, the recovery of certain metal values has become economically feasible. Particularly in cases where the sludge has a high content of organic matter, elimination of the organic matter is a major step early in the over-all process of treating the sludge and concentrating the mineral values. A number of different dewatering approaches are used, including physical separation and/or drying of the sludge in sunlight in arid climates. In some cases it is economical to treat the sludge directly with mineral acids to degrade the organic matter, but usually the cost of direct acid treatment becomes prohibitive because of the cost of the quantity of acids required to fully break down the sludge in this manner.
The usual way of degrading the organic matter in the sludge is by incineration, and this treatment is frequently performed on sludges having a high content of silicates. It is to this type of treatment of sludges that the present invention is directed.
Incineration of sludges is generally carried out at high temperatures, of the order of 1800 degrees F and above, using specially built multiple hearth furnaces which achieve high enough temperatures to rapidly destroy odors and pathogens. However, in the case of sludges having substantial silica content these high temperatures tend to induce reactions between the silica and contained metal salts, which reactions lead to the formation of glass matrices which occlude metal values inside their cellular walls. This is a very serious disadvantage because it renders the occluded metal values virtually inaccessible to conventional hydrometallurgical extraction processes, whereby significant proportions of the total metal values in the sludges do not become separated by acid leaching of incinerated sludge ash. Processes of the type in which metal values become bound in silica glass matrices are typically found in U.S. Pat. No. 4,033,763 to Markels, particularly including processes wherein recovery of metal values from the ash is done by hydrometallurgy and by cyanide extraction.
The deleterious effect of this phenomenon on efforts to recover metal values from sludge ash is not well recognized in the prior art. In U.S. Pat. No. 3,974,783 to Flynn, the inventor discusses the fact that the formation of slag during high temperature sludge incineration is damaging to incinerator parts on which the slag is formed. This patent therefore sets out to reduce the amount of slag formed, and to make that slag which is formed softer and more easily breakable from the incinerator parts. This is accomplished by adding certain metals to the sludge including copper, cobalt, manganese, iron or calcium which lowers the ignition temperature of carbon, and by adding magnesium compounds to the sludge prior to incineration. However, this patent is not directed toward preventing the formation of glass matrices in the ash which tend to occlude metal values and thereby reduce the effectiveness of subsequent hydrometallurgical treatment of the ash.
U.S. Pat. No. 4,133,273 to Glennon mixes other waste materials with sludges so that the mixed materials help fuel each other during incineration, but there is no discussion of the effect of slagging due to high temperature incineration.
The metal values present in the sludge can be classified in three different groups as follows:
(1) the noble metals including Ag, Au, Pd and Pt;
(2) the base and transition metals including but not restricted to Al, Fe, Cu, Cr, Sn, Ga, Zn, Mn, Tl, Cd, Co, Mo, etc; and
(3) the ions typically of sulfate, nitrate, and phosphate, which ions are also extracted during the acid leach step, and can be recovered by suitable ion exchange techniques.
Extraction of the metals of these groups can be efficiently achieved by leaching the ash directly with hot acids, such as nitric, hydrochloric and sulfuric, to separate the content into dissolved metal values and residue of the ash which is mainly silica but includes the noble metals also. Final extraction is then achieved by performing such other hydrometallurgical recovery steps as may be necessary to isolate and recover individual metals. However, those metal values which have become occluded inside insoluble glass matrices will generally remain inside the glass matrices and be lost with the residue because they do not respond to subsequently performed acid leach and hydrometallurgical recovery steps. The present invention addresses itself to the steps required to prevent formation of such glass matrices, whereby substantially all of the metal values can be recovered from the ash without a significant percentage thereof being inaccessible to such further processing. Typical hydrometallurgical recovery steps are outlined in examples presented in this specification, although these steps are not, of themselves, considered novel.